I've created a script to auto-generate the NP++ userDefineLang.xml from a "sourcepawn_include" directory. It also has some of the newer NP++ highlighting options than the one on the first post (such as recognizing an escape character in strings).
You can generate an xml file from your own include dir, such as one with incs from extensions you may use and/or one from a different SM version.
Here is a link to the script and the userDefineLang.xml for a standard SM 1.3
http://hg.alliedmods.net/sourcemod-central/file/tip/editor/npp